The present invention relates to apparatus for producing a specific flow of bulk material from a storage container. By bulk material is meant particulate materials having particle sizes which range from powder granules to lump goods. The apparatus falls in the category of metering belt feeders. Such belt feeders comprise a conveyor belt and a funnel, or hopper, from which the material is drawn or from which the material runs onto an endless belt, and on which the material is weighed on a given part of the belt, prior to leaving the belt.
The flow of material leaving the belt is controlled by adjusting the supply of material thereto or by adjusting belt speed.
The most difficult problem associated with belt feeders is one of guiding the side-edges of the belt. This is primarily because the length of the belt conveyor generally is provided to be as short as possible, for process reasons of a technical nature. The material transported on initial parts of the belt causes changes in the geometry which provides stable belt movement in the starting position, due to which, after a time, the belt tends to wander sideways until it comes into contact with an obstruction of some kind or another. Lateral displacement of the belt, and particularly lateral displacement to such extreme positions, results in weighing errors. The problem is accentuated by the fact that a low belt tension is desired, so that the material can be weighed effectively.
It is known that this problem can be alleviated by cambering the drive roller and by providing the roller with a high friction surface. Soiling of the belt and changes in belt properties due to changes in temperature and also to wear, necessitate constant monitoring of the conveyor belt.
Also known to the art are mechanisms which influence lateral distribution of belt tension in an attempt to compensate for wandering of the belt. Such mechanisms are complicated and become soiled or dirty and therefore require constant inspection.
With the intention of reducing belt tension over that part of the belt which moves over the weighing path, it is known from FR-A-2 145 366 to position the belt drive means between that part of the belt onto which the material is drawn or runs out onto the belt and that section of the belt on which the material is weighed.
The position of the belt drive means presumes that the belt is caused to move over a longer, substantially horizontal plane in the weighing path than the remainder of the belt onto which the material is deposited. The belt is caused to form a loop between those planes with the aid of at least three guide rollers, the loop being so configured as to be able to draw tire material supplied to the belt down to the lower belt plane. A guide roller located at the lowest hang of the belt loop has the form of a belt-tensioning roller and one of these rollers functions to drive the belt. The tension roller imparts to the belt sufficient tension for the friction generated by the drive roller against the belt to drive the belt around the loop, and also determines the tension in that part of the belt which is located above the weighing path. These two forces produced by the tensioning roller are equally as large, and in order to prevent the belt from slipping on the drive roller, a significant degree of tension is induced in the belt, and even in the belt-section that is located over the weighing path, even though this tension is lower than the tension in that part of the belt which runs on the higher level. The fact that the transported material is caused to fall freely between the height levels means that collections of the material, and therewith coatings of material on the rollers and on the belt, will render positive function impossible with regard to belt movement and to weighing accuracy, a drawback which is particularly manifest in the case of small material flows which a low belt speed must be maintained in order to achieve weighing accuracy. The drive arrangement does not therefore provide a solution to the problem of belt control.